1. Field of the Invention
The present invention relates to proximity sensors and more particularly to a proximity sensor system in which the measurement of the distance between the sensor and the metal target is made insensitive to noise, changes in the temperature of the sensor, and different lengths of wire thereto,
2. Description of the Prior Art
U.S. Pat. No. 5,180,978 of Postma et al issued Jan. 19, 1993 and assigned to the assignee of the present invention (hereinafter the 978 patent), describes a two wire proximity sensor having a sensing coil movable into proximity with a metal member or target and provides for the direct measurement of the AC and DC resistances of the coil and, in one embodiment the imaginary impedance component i.e. the reactance. A microprocessor utilizes these values and provides an output indicative of the distance between the coil and the target. The measurement of the distance to the target affects the difference between the AC resistance and the DC resistance and this difference does not vary much with temperature. Since the wire has equal values of AC resistance and DC resistance, the effect of the wire is cancelled by taking the difference of these two resistances.
One difficulty has been encountered in the use of the invention of the 978 patent and that occurs when the lengths of the wires to and from the sensor become long enough that the capacitance introduced thereby causes phase shifts which prevent the measurement of the AC resistance of the sensor. Although using low frequency can minimize this effect, attempts to make this effect extremely small motivate the use of a frequency so low that the eddy currents in the metal target no longer produce the desired change in the AC resistance. Aircraft manufacturer requirements have been changed from requiring plus or minus 1.0 mm accuracy to requiring an accuracy of plus or minus 0.1 mm and to requiring that the measurement be insensitive to temperatures between xe2x88x9277xc2x0 C. to +125xe2x88x92xc2x0 C. with cable lengths of 3 m to 80 m in an unshielded twisted pair.
The present invention overcomes some of the problems associated with the new accuracy requirements and allows great accuracy over extreme cable length variations with substantially no problems with noise, sensor temperature variations and capacitance. The basic change over the prior systems is to utilize DC and AC conductance and the quadrature component of AC conductance i.e. susceptance. An automated test is then set up to record these values for the various cable lengths over the full temperature and the target gap ranges to create a data base. Thereafter, the gap may be calculated using an equation that makes use of the coefficients derived from said database. Accuracies of plus or minus 0.1 mm are easily obtainable over the entire range of cable lengths and sensor temperatures.